<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box1{
        width: 0px;
        height: 0px;
        border: 10px red solid;
        border-top: none;
        border-color: transparent transparent blue transparent;
    }
</style>
<body>
    <div class="box1"></div>
</body>
</html>